Thousands of Pakistani opposition supporters have joined a large convoy headed to the capital, Islamabad, for a key rally to demand the ouster of the prime minister over allegations of vote fraud.

The rally is seen as the strongest challenge yet to the government of Prime Minister Nawaz Sharif, just a year after he took office in the first democratic transfer of power in a country long plagued by military coups.

The protesters set out this morning from the city of Lahore in cars, trucks and buses, while others walked or drove motorcycles as they embarked on the 187 miles (300km) journey to Islamabad. Initially 5,000 protesters were on the march, but the number steadily increased. The convoy was moving so slow that it could cover only a distance of three miles (5km) in seven hours.

The convoy is led by Imran Khan, famous cricketer-turned-politician who heads the Tehrik-e-Insaf party, the third-largest in parliament. The demonstration was called to coincide with Pakistan's Independence Day.

"Get ready to win the match," said Mr Khan, who is revered in Pakistan for leading the national team that won the 1992 cricket World Cup.

Mr Sharif's ruling party has rejected Mr Kahn's demand, saying the elected democratic government would complete its term, which ends in 2018.

Thousands of policemen were deployed across Islamabad and along the convoy's route while the capital's entry points were blocked since earlier this week with large shipping containers. "We are taking measures to secure the capital from any violence," said police official Jamil Hashmi.

Earlier, the convoy got off to a colourful start in Lahore, with protesters dancing to the beat of the drums and singing patriotic songs. Many women had the green and white of the Pakistani national flag painted on their cheeks, along with the red and green of Khan's party.

Also on the march was Tahir-ul-Qadri, a Pakistani cleric who is also a Canadian national and who commands a loyal following of thousands through his network of mosques and religious schools in Pakistan. He left Lahore with thousands of his followers and was expected to join Khan's rally on the road or in Islamabad.

Both the cleric and Mr Khan demand Mr Sharif's government step down and new elections be held. Mr Khan alleges last year's vote was invalid due to widespread rigging by government supporters.

There was also concern that, once in Islamabad, the rally could descend into violence. "We are out on the streets to do our struggle for a change in the system," said one of the protesters, Mohammad Faheem.

Pakistan, a nuclear-armed country of 180 million people, has largely been ruled by military dictators since it was carved out of India in 1947.

Mr Sharif, himself overthrown in the 1999 coup that brought former army chief Pervez Musharraf to power, has met regularly with top advisers ahead of the rally. The government has also invoked a rarely-used article in the constitution allowing the military to step in to maintain law and order if needed.

Speaking at an Independence Day ceremony in south-western Pakistan, Mr Sharif criticised the opposition rally, calling it "negative politics".

Mr Sharif said Mr Khan would be better advised to "work to alleviate poverty and improve law and order" in Pakistan.